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Topic Review (Newest First)

08-12-2014 11:31 AM

oldbogie

Quote:

Originally Posted by v8hed

383 with GM HEI distributor (modded with aftermarket module, curve kit, coil and shimmed for end play). Before going to the chassis dyno, I'd set my timing up (with a good timing light) for 34* total. Checked it up to about 4,000rpm and I could see the advance curve adding more timing until around 3,000rpm then it remained static. As I said, I only went up to about 3,500-4,000rpm with the timing light... just enough to see that the advance curve had finished adding timing. On the chassis dyno, we were messing around with the timing (just adding/subtracting a couple of degrees to do some fine tuning). I asked the dyno operator to take it up to 6,000rpm and keep the timing light on the balancer. He said total timing went up to something like 50* by 5,500rpm! We checked with another light and same thing. Couldn't hear any pinging (although the car is v.loud). What I don't understand is how could this possibly be happening? I still don't believe everything is at it seems, since taking timing out resulted in less power. If is was really running up around 50* total timing, there's no way the motor would be making power. I've previously tried experimenting locking the mech advance out and didn't see over 36* (with about 14* initial). Since the holes in the base plate physically limit max advance, what's going on with this wacky timing reading? It had occurred to me that perhaps the ProStreet balancer I'm using could be slipping, but it's pretty new and looks perfectly fine to the naked eye.

Anyone got any other ideas or explanations? If not, my next step will be to try a brand new MSD HEI just to eliminate the distributor from the equation. What about spark scatter or some other kind of effect?

If you're running vacuum advance, which you are, you have to go further than just pulling the vacuum can hose and plugging the port the advance plate also needs to be screwed down so it cannot move, otherwise its position is simply dependent upon the return spring in the vacuum can which can be excited by all the vibrations going on when the engine is running at high RPMs.

Another common event is the natural looseness in the timing chain and how it reacts to high RPM excitement. Then there are a number of clearances such as the clearance between the cam's distributor gear and the distributor's gear. The thrust clearance of the cam, typically the flat tappet chevy is unrestrained in forward thrust except by the distributor gear and there is a clearance of the distributor shaft and the housing allowing it some freedom of movement. Even roller cams have a little thrust freedom that works on the distributor gear mesh. All cams have a lot of rotational shake from the slowing when valves are opened against their springs to accelerations on the closing side. Nothing in this system is truly stable. All these clearances can and do gang up to change the timing. This is the reason why competition engines use the crankshaft as the timing reference to trigger the spark rather than a distributor at the end of a group of gears, chains and shafts. There are also other gadgets sold that apply tension to the timing chain for a more consistent holding of set position by not allowing any slack in the chain. Not as good as a crank trigger for ignition timing but very helpful in keeping the camshaft to crankshaft alignment steady. A thrust button on a flat tappet cam is also helpful to limit its fore and aft travel so that doesn't disturb the distributor gear mesh with the cam. Gear drives can be useful in eliminating the gear chain issues but you buy something of the quality and design of the Shaver-Wesmar to really get the needed effects the popular inexpensive drives don't solve this problem and make a lot of noise not doing it.

Bogie

08-11-2014 08:47 PM

BuzzLOL

.
. Sounds to me like the carburetor was too small and high RPMs vacuum was pulling up the vacuum advance a bit...

07-09-2013 11:41 PM

kc8oye

not to be a smart ***** or anything... but did he have the light on the right ignition wire??

I ask this because I went down this road once... when my ignition wires were a rats nest, I missed the #1 wire and got the #2 wire instead.. couldn't figure out why my timing was WAYYY off, and when it got it where i thought it should be, the engine wouldn't run....

spent better part of an hour banging my head against the wall on that one..

10-28-2012 11:32 PM

Greg T

Today, I use the MSD E-Curve. Worth every penny.

.

10-28-2012 06:30 PM

Old Fool

back in the day a distributor was put on a distributor machine and had its advance curve set. no guessing, no oh ****s, you knew what you had. That was before they became dizzy's.

04-09-2012 05:14 PM

2x4 Barrels

to 10

Hey 10

Check out these bowls out.of course you have look at them upside down

2x4`s

04-08-2012 11:25 AM

Custom10

Quote:

Originally Posted by cool rockin daddy

Can somebody please translate the above gibberish?

I see the word bowls used allot, I am thinking the bowls are ablaze

04-07-2012 01:02 PM

T-bucket23

If you want to prove if it is mechanical or electronic it is simple and has been stated in this thread several time.
Disconnect the vacuum advance
Lock the timing at something reasonable and see if it still jumps.
If you lock it and it still moves then it is not the advance weights or related parts. I would then look at the module or pickup. Either could cause the issue. A bad pickup may not be up to the rpm task and could be collapsing the field a little to soon at higher rpm or not building the field long enough. I would also check the air gap on the pickup it becomes more critical at higher rpm. The module could also be an issue but the pickup would be my first guess if it is electronic.
I think if the timing chain was stretching you would be loosing timing not adding and it would be inconsistant.

04-07-2012 12:43 PM

2x4 Barrels

spark

Hi guy`s

Is it possible that voltage rise time is the real issue.What I mean is hei pick up voltage will rise with r.p.m. and alter timing.

2x4`s

04-07-2012 08:43 AM

cool rockin daddy

Can somebody please translate the above gibberish?

04-07-2012 02:18 AM

robertalive

ignition

no its just if the cam was dialed you have advanced the engine.so the ignition is just tuning the spark timing.i was allways taught the distributor is the moved bout an inch to a inch and a half prior to the spark of the .so if the #1 spark connection on the distributor is moved to spark bout an inch or so of travel before it lines with its own cable headed out to the #1 spark plug.so the spark is fired then by the time it shoots out to the spark plug ,it has occured ,if it went off at the same time of the distributor #1 being lined up to spark plug #1 it would be to late and the engine wouldnt run.i didnt understand the timing in that engine changing as the rpm raised.i allways buy the best i can afford in equipment and follow the instructions and research on you tube and internet and never once ran into that problem.i helped build a 65 mustang we built and now did a chevy 350 vortec 4 bolt main ,1 piece rear seal.just a 350 sbc but never bore to 60 for heat problems,i try to stay at .030 maybe .040 but even that i dont know cause you alter one part to much and then it starts throwing you Off in other areas.so i just port and polish the heads maybe 1mm off the exhaust chambers to wake it up,dont want my headers overlapping a 1.5 or 1 5/8 primariey.i really get technical bout the bowls ill just buy a pair of junk valves and port like any of us would go ahead and do.want he bowls like mirrors,mirrors i tell ya.lol .hope that cleared it up a little.sorry for confusing the masses.lol.if it makes sense and you wanns motor chat with ,hit me back anytime.bye dude

04-06-2012 12:14 AM

4 Jaw Chuck

Another explanation is called spark scatter, high cylinder pressure causes a higher load on the ignition system which can cause the coil to delay or jump across the plug gap only after the mixture self detonates.

High compression ratios combined with weak stock style ignition systems are a deadly combination, my bet is this phenomena you are seeing only occurs under load above a certain rpm...if so you are getting a clue of your ignition system weakness.

A good test is to physically lock the timing at a high initial like 18 degrees and repeat the test on the dyno, if your still seeing it assume your ignition system isn't up to the task and needs upgrading.

04-05-2012 09:49 PM

cobalt327

Huh????? I read that twice and still have no idea on 90% of it.

04-05-2012 09:09 PM

robertalive

not the answer but feedback

i usually advance the cam then fine it with the distributor ,minor.it would make you lower the timing to hit 34 at 6000 rpm.that would hurt you low.but maybe youll have to retrace youre steps to the point of failure and adjust from there.i run a HEI with sbc vortec 350.dot to dot on the cam on comp cam mother thumper is fine 2-4 degree advance there or go alittle more.so i dont really want to have to do much more than an 1" to 1.5"advance manually on the dis.like the part bout mech advance only affecting low drive to an idle imma keep that in mind for when i next tune.love carbed out monsters and its true fine tuning and constant adjust comes with the territory.good luck with it brother.out

04-05-2012 08:19 PM

dbrow6272

tIMING

So no load means they set the timing running around the trackk? No seriously, this can be done on a dyno and I'm thinking it probably is although I haven't been near a dyno in just about as many years. I stick by the vacuum method and have used it on many hot rodded engines. Then I have used a light with an advance dial on it to "fine tune" timing. I do not trust all balancers, some yes, but not all. Many times I have found the timing ring on a balancer slipped or plain just improperly installed. Use the vacuum gauge first, check the timing mark to see where the balancer is and go from there.

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